1. Field of the Invention
The invention relates generally to the production of liquid fuels from aquatic biomass, and more particularly to improved processes for producing aquatic biomass and for converting aquatic biomass to bio-oil, which can be used as a burning fuel, or in turn can be refined to fuels for internal combustion engines.
2. Description of the Related Art
Aquatic plants, such as algae, are a source of lipids, such as triglycerides and aliphatic hydrocarbons; carbohydrates, such as lignin; and proteins.
The advantage of using micro algae is that they have very high growth rates, utilize a large fraction of the solar energy (up to 10% of the solar energy), and can grow in conditions that are not favorable for terrestrial biomass growth.
The U.S. Department of Energy funded a program to develop renewable transportation fuels from algae, and the results of this program are reported by Sheehan et al. [1] Over 3000 strains of micro algae were collected as part of this program. Micro algae are one of the most primitive forms of plants and are microscopic photosynthetic organisms. While the photosynthesis mechanism in algae is similar to other plant material, they can convert much more of their solar energy into cellular structure (up to 10% instead of maximum 1% by terrestrial sources).
Macro algae are commonly known as seaweed. Both micro algae and macro algae are fast-growing marine and freshwater plants. Commercial production of triglycerides from micro algae has been estimated to be 72 000 L/ha-year (390 boe/ha-year), and it has been estimated that rates as high as 130 000 L/ha-year (700 boe/ha-year) could be accomplished.
This means that algae have triglyceride production rates 45-220 times higher than terrestrial biomass. Other estimates indicate that 2000 ha of land would be required to produce 1 EJ/year of fuel with micro algae. For comparison, the U.S. consumed 42 EJ of petroleum products in 2003.
Micro algae are categorized into four major classes in terms of their abundance: diatoms, green algae, blue-green algae, and golden algae. Micro algae can contain from 7 to 60 dry wt triglycerides.
Pilot plant tests [2], conducted over a six-year period, demonstrate that micro algae could be produced at productivity rates as high as 500 kg algae/ha in a 1000 m2 pond for a single day. The ponds were an open face shallow water design where the water and algae are circulated around the pond. Nutrients and CO2 were continually added to the algae pond. The productivity was dependent on temperature and sunlight, which varied over the course of the experiments. Ideally, algae could be produced on algae farms in open, shallow ponds where a waste source of CO2, for example, from a fossil fuel power plant, could be efficiently bubbled into the ponds and captured by the algae.
The current limitation of micro algae is the high production cost. The total biomass algae cost is in the order of 200 to 300 $/metric ton, which is considerably higher than the cost of lignocellulosic biomass (less than $40/metric ton). The cost for CO2 is 20-30% of the total cost, and using waste CO2 from fossil fuel power plants would decrease the cost of algae production.
The conclusions from the cost analysis [1,2] is that alternative engineering designs for micro algae production would not significantly reduce the cost of micro algae production. The limiting factor in cost analysis is micro algae cultivation issues, and according to Sheehah [1] future research work should focus on the biological issues regarding micro algae production. Micro algae cultivation issues are limited by the availability of water, CO2, sun light, and flat land. The development of low-cost harvesting processes can also significantly reduce the cost of algae.
Thus, there is a particular need for improving the efficiency of the conversion of aquatic biomass to bio-oil.